ruby: remove unsupported versions (#34927)

* remove EOL ruby versions for security and maintenance reasons.
* only expose ruby_MAJOR_MINOR to the top-level. we don't provide
guarantees for the TINY version.
* mark all related packages as broken
* switch the default ruby version from 2.3.x to 2.4.x

pkgs/applications/misc/taskjuggler/default.nix

@@ -7,6 +7,7 @@ bundlerEnv {
   gemdir = ./.;
 
   meta = {
+    broken = true; # needs ruby 2.0
     description = "A modern and powerful project management tool";
     homepage    = http://taskjuggler.org/;
     license     = lib.licenses.gpl2;